加载过程

首先来看启动类，启动类上包含了@SpringBootApplication注解

@SpringBootApplication
@MapperScan("com.cskaoyan.mapper")
public class Application {public static void main(String[] args) {SpringApplication.run(Application.class, args);}
}

在@SpringBootApplication注解中包含了@EnableAutoConfiguration注解

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = {@Filter(type = FilterType.CUSTOM,classes = {TypeExcludeFilter.class}
), @Filter(type = FilterType.CUSTOM,classes = {AutoConfigurationExcludeFilter.class}
)}
)
public @interface SpringBootApplication {

在@EnableAutoConfiguration中包含了@Import({AutoConfigurationImportSelector.class})，通过Selector选择器找到对应的自动配置类

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import({AutoConfigurationImportSelector.class})
public @interface EnableAutoConfiguration {

在AutoConfigurationImportSelector中包含了selectImports方法

public String[] selectImports(AnnotationMetadata annotationMetadata) {if (!this.isEnabled(annotationMetadata)) {return NO_IMPORTS;} else {AutoConfigurationImportSelector.AutoConfigurationEntry autoConfigurationEntry = this.getAutoConfigurationEntry(annotationMetadata);return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());}
}

然后再看selectImports方法里的this.getAutoConfigurationEntry(annotationMetadata)，在该方法中包含这样的一行代码，获得配置类的字符串的List

protected AutoConfigurationImportSelector.AutoConfigurationEntry getAutoConfigurationEntry(AnnotationMetadata annotationMetadata) {if (!this.isEnabled(annotationMetadata)) {return EMPTY_ENTRY;} else {AnnotationAttributes attributes = this.getAttributes(annotationMetadata);//这一行就是获得配置类(全限定类名)的List信息List<String> configurations = this.getCandidateConfigurations(annotationMetadata, attributes);configurations = this.removeDuplicates(configurations);Set<String> exclusions = this.getExclusions(annotationMetadata, attributes);this.checkExcludedClasses(configurations, exclusions);configurations.removeAll(exclusions);configurations = this.getConfigurationClassFilter().filter(configurations);this.fireAutoConfigurationImportEvents(configurations, exclusions);return new AutoConfigurationImportSelector.AutoConfigurationEntry(configurations, exclusions);}
}

进入到getCandidateConfigurations方法中

protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {// 加载/META-INF/spring.factories文件中的自动配置类List<String> configurations = new ArrayList(SpringFactoriesLoader.loadFactoryNames(this.getSpringFactoriesLoaderFactoryClass(), this.getBeanClassLoader()));// 加载/META-INF/spring/org.springframework.boot.autoconfigure.AutoConfiguration.imports文件中的自动配置类ImportCandidates.load(AutoConfiguration.class, this.getBeanClassLoader()).forEach(configurations::add);Assert.notEmpty(configurations, "No auto configuration classes found in META-INF/spring.factories nor in META-INF/spring/org.springframework.boot.autoconfigure.AutoConfiguration.imports. If you are using a custom packaging, make sure that file is correct.");return configurations;
}

要看/META-INF/spring.factories接下来进入到loadFactoryNames这个方法中

public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) {ClassLoader classLoaderToUse = classLoader;if (classLoader == null) {classLoaderToUse = SpringFactoriesLoader.class.getClassLoader();}String factoryTypeName = factoryType.getName();return (List)loadSpringFactories(classLoaderToUse).getOrDefault(factoryTypeName, Collections.emptyList());
}

最后来看loadSpringFactories这个方法，这个方法的返回值为Map，这个Map的key为字符串，value为字符串列表List，这个key其实就是需要EnableAutoConfiguration，value就是自动配置类的字符串List信息

private static Map<String, List<String>> loadSpringFactories(ClassLoader classLoader) {Map<String, List<String>> result = (Map)cache.get(classLoader);if (result != null) {return result;} else {HashMap result = new HashMap();try {///META-INF/spring.factoriesEnumeration urls = classLoader.getResources("META-INF/spring.factories");//略掉很多代码}}
}

到这里大家其实可以看到最终加载的就是/META-INF/spring.factories文件

要看/META-INF/spring/xxx.imports

public static ImportCandidates load(Class<?> annotation, ClassLoader classLoader) {Assert.notNull(annotation, "'annotation' must not be null");ClassLoader classLoaderToUse = decideClassloader(classLoader);///META-INF/spring/org.springframework.boot.autoconfigure.AutoConfiguration.importsString location = String.format("META-INF/spring/%s.imports", annotation.getName());Enumeration<URL> urls = findUrlsInClasspath(classLoaderToUse, location);ArrayList importCandidates = new ArrayList();while(urls.hasMoreElements()) {URL url = (URL)urls.nextElement();importCandidates.addAll(readCandidateConfigurations(url));}return new ImportCandidates(importCandidates);
}